The present invention relates to an acoustic communication system. More specifically, the present invention relates to an acoustic, multi-channel, digital communication system for use in a liquid medium.
The difficulties of acoustic digital communication in a liquid medium are well known. Multi-path interference results from reflected signals. Such multi-path interference along with environmental noise, temperature variations and other factors negatively impacts reliable communication. Further, for applications requiring multi-channel communications without cross channel interference, such requirements add to the difficulty of reliable communication.
Several devices and methods have been disclosed by the prior art to include the use of multiple carrier frequencies, matched filtering, automatic gain control, serial correlation and relatively high transmit power to address signal distortion caused by noise and multi-path interference. Generally speaking, these methods carry the disadvantages of relatively high implementation costs and often less than desired reliability. For example, the use of multiple carrier frequencies requires acoustic transducers that have sufficient bandwidth to accommodate the frequency range. This approach also requires exceptionally tight filtering to prevent signals on a given frequency from interfering with signals using other frequencies. The resulting bandwidth and filtering requirements create higher component and implementation costs. The use of relatively high transmit power to differentiate true signals from noise and reflected signals also has the disadvantage of higher component and operating costs. Typical of the art are those devices disclosed in the following U.S. Patents:
Of these patents, Isaak et al., (""164) teach an improved sonar system with exceedingly long range that is not readily susceptible to enemy detection. The ""164 device is operated by generating a pseudo-noise output signal using multiple carrier frequencies decoded by an equivalent receiver.
The ""147 device taught by Andrews et al., is an acoustic, underwater telemetry system that uses a combination of short and long pseudo-noise codes across multiple carrier frequencies.
Brady et al., (""207) teach an underwater local area network that seeks to avoid collision from multiple transmissions by using a virtual circuit system. The virtual circuit system routes a signal through a series of transmitters and receivers based on node sequences established by the network nodes.
Cafarella et al., (""620) teach a high data rate acoustic communication system that encodes, transmits, receives and decodes a digital signal over a carrier frequency. The method steps used by the ""620 device include generating a spread spectrum waveform in real time for the transmit signal; using phase-based keying; conditioning with a power amplifier and automatic gain control (AGC); using antipodal, bi-phase modulation; conditioning the received signal using AGC; performing matched filtering on the received signal; performing signal detection using an amplitude based trigger signal; and performing serial correlation on the received signal. The ""620 device incorporates a transmitter which includes a spread spectrum waveform generator, a heterodyne state, and a power amp. The spread spectrum waveform generator includes a feedback shift register sequence generator and a means to create a digital power control word, each operating in real time. Further, the waveform generator includes, among other things, an FIR digital filter, digital-to-analog conversion circuitry, spectral shaping, an up-conversion stage and a differential power amplifier driver.
Woodsum et al. (""339) teach an underwater location and communication system that utilizes a method of encoding where each data bit in a block of data occupies a separate and distinct frequency band.
An object of the present invention is to provide a device for acoustic communication of coded multi-channel digital messages in a liquid medium using a single carrier frequency.
Another object of the present invention is to provide high reliability of communication by dealing effectively with multi-path interference and noise conditions found in liquid environments.
Still another object of the present invention is to provide communication between transceiver modules.
Yet another object of the present invention is to provide multi-channel capability that will allow communication among transceiver modules on one designated channel to operate without interference from other modules operating on another designated channel.
Another object of the present invention is to accomplish such a device at a lower implementation cost when compared to prior art devices.
Still yet another object of the present invention to provide such a device which operates using lower power when compared to prior art devices.
A further object of the present invention is to provide peripheral interfaces to allow information to be communicated to and from a variety of peripherals.
Another object of the present invention is to provide a host interface that allows interaction between the present invention and an intelligent host.
Another object of the present invention is to provide a user interface that allows interaction between the present invention and a variety of user communication and control devices.
Other objects and advantages will be accomplished by the present invention which serves to accomplish acoustic communication of coded multi-channel digital messages in a liquid medium at lower cost and using lower power than prior art devices. The device is designed to use a single carrier frequency. The device includes the use of acoustic transducers in signal communication, and may include interfaces for communication with a variety of peripherals, host computing devices and/or user communication and control devices. The device of the present invention incorporates effective methods of encoding, transmitting, receiving, decoding, and acknowledgment of digital signals for achieving the desired level of reliability, bi-directional communication and multi-channel capability.
The device is a communication subsystem capable of encoding, transmitting, receiving and decoding a digital message and a message acknowledgment, and is configured to provide such communication with another device of the present invention. The device is composed primarily of electronic circuitry containing a processor, system firmware and other electronic components necessary to enable system operation. The device includes at least one acoustic transducer to send and receive the acoustic signals using one designated carrier frequency. The device may include a peripheral interface that allows communication with a variety of peripherals. A host interface may be carried on the device for allowing communication with an intelligent host device such as a personal computer. Further, the device may include a user interface for allowing interaction with a variety of user communication and control devices such as alarms, push buttons, and displays.
The elements contained within the electronic circuitry, in conjunction with the acoustic transducers, collectively perform message encoding, transmission, reception, decoding and acknowledgment, and may utilize the functions of the peripheral interface, host interface and user interface. Specifically, with respect to message encoding, the message is encoded for transmission, including at least a message definition, a message header, and error detection and/or correction data. With respect to message transmission, the present invention addresses chip configuration, multi-channel operation, inter-chip gaps, message modulation, and single carrier frequency. Similarly, with respect to message reception, the present invention addresses single carrier frequency, signal conditioning, filtering, frequency mixing, compression, sampling, decompression, noise floor removal, and digital filtering. Message decoding addresses chip array averaging, shape based correlation, correlation coefficient averaging, message header verification, payload and error detection and/or correction, expected bit verification, and optionally, time-shifted parallel decoding and detection. Finally, with respect to message acknowledgment, the present invention provides a confirmation message and follows retransmission protocol that is specified for the application.